In recent years, organic EL devices have been under intensive investigation. One such organic EL device basically includes a transparent electrode (a hole injecting electrode) of tin-doped indium oxide (ITO). A thin film is formed on the transparent electrode by evaporating a hole transporting material such as triphenyldiamine (TPD). A light emitting layer of a fluorescent material such as an aluminum quinolinol complex (Alq.sup.3) is deposited on the hole transporting thin film. An electrode (an electron injecting electrode) is formed thereon from a metal having a low work function such as magnesium or Mg. This organic EL device attracts attentions because they can achieve a very high luminance ranging from several hundreds to tens of thousands cd/m.sup.2 with a voltage of approximately 10 volts.
An electron injecting electrode considered to be effective for such organic EL devices is made up of a material capable of injecting more electrons into the light emitting layer or electron injecting and transporting layer. In other words, the lower the work function of a material, the more suitable is the material as the electron injecting electrode. Various materials having a low work function are available. Regarding materials used as the electron injecting electrode of organic EL devices, for instance, JP-A 2-15595 discloses an electron injecting electrode comprising a plurality of metals other than an alkali metal, at least one of which has a work function of less than 4 eV, typically MgAg.
A preferable material having a low work function is an alkali metal. U.S. Pat. Nos. 3,173,050 and 3,382,394 disclose NaK as one example of the alkali metal. However, an electron injecting electrode made up of the alkali metal is inferior to that built up of MgAg, etc. in terms of safety and reliability, because the alkali metal has high activity and so is chemically unstable.
To use such a material having a low work function as mentioned above or its alloy, film sealing has been under investigation. To achieve sufficient sealing, however, it is required to make use of costly yet troublesome sealing films such as those of Teflon or SiO.sub.2 because any desired result is unachievable by use of glass sealing or the like. Such film sealing should be carried out immediately after the formation of an electron injecting electrode, thereby preventing corrosion of the electron injecting electrode by oxidation. To this end, however, dedicated film sealing equipment should be provided separately.
In efforts to prevent oxidation, it has been proposed to form an aluminum cap layer on Mg.Al as set forth in JP-A 4-233194 or to use a cap layer formed of an alkaline earth metal, a rare earth metal or the like which have a work function much lower than that of the electron injecting electrode material. However, such cap layers have for its object the prevention of appearance of dark spots, and so serve mainly to absorb moisture at an interface between a cathode (electron injecting electrode) and an organic electroluminescent medium, etc. In other words, these cap layers are less than satisfactory for a sealing film protecting the whole of an organic EL device.